Many types of input devices are presently available for performing operations in a computing system, such as buttons or keys, mice, trackballs, joysticks, touch sensor panels, touch screens and the like. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation as well as their declining price. Touch screens can include a touch sensor panel, which can be a clear panel with a touch-sensitive surface, and a display device such as a liquid crystal display (LCD) that can be positioned partially or fully behind the panel so that the touch-sensitive surface can cover at least a portion of the viewable area of the display device. Touch screens can allow a user to perform various functions by touching the touch sensor panel using a finger, stylus or other object at a location often dictated by a user interface (UI) being displayed by the display device. In general, touch screens can recognize a touch and the position of the touch on the touch sensor panel, and the computing system can then interpret the touch in accordance with the display appearing at the time of the touch, and thereafter can perform one or more actions based on the touch. In the case of some touch sensing systems, a physical touch on the display is not needed to detect a touch. For example, in some capacitive-type touch sensing systems, fringing electrical fields used to detect touch can extend beyond the surface of the display, and objects approaching near the surface may be detected near the surface without actually touching the surface.
Some capacitive touch sensor panels can be formed by a matrix of plates (e.g., touch electrodes) made of conductive materials (e.g., Indium Tin Oxide (ITO)) and coupled to routing traces made of a conductive material (e.g., copper). In some examples, plates and routing traces may be formed of the same conductive material. In some examples, a routing trace may comprise a first portion made of a first conductive material (e.g., ITO), and a second portion made of a second conductive material (e.g., copper), which in some examples may be overlaid onto the first conductive material. For instance, a first portion of a routing trace overlapping a viewable area of a display may be made of a transparent conductive material (e.g., ITO), such that the viewable area is visible through the first portion, while a second portion of the routing trace extending outside the viewable area may be made of an opaque conductive material (e.g., copper).
Some touch screens can be formed by at least partially integrating touch sensing circuitry into a display pixel stackup (i.e., the stacked material layers forming the display pixels). However, touch electrodes and routing traces can be susceptible to noise from above and/or below the touch sensor panel. For example, environmental noise, including capacitive coupling between objects above a touch sensor panel, such as human fingers, and routing traces, may interfere with proper operation of the touch sensor panel. Similarly, the display circuitry in a touch screen, which in some examples can be positioned below a touch sensor panel, may present noise that interferes with the ability of the touch sensor panel to detect changes in capacitance. It is desirable to shield touch electrodes and routing traces from noise from above and/or below the touch sensor panel. Some touch sensor panels accomplish this with a laminate including two layers of a substrate material (e.g., cyclo olefin polymer), with conductive materials applied to each of the two substrate layers. Fabricating a touch sensor panel using such a two-substrate structure can be costly and complex. Additionally, each substrate contributes to the thickness of the touch sensor panel. It is desirable to reduce the cost and complexity of fabricating touch sensor panels, and also to reduce the thickness of touch sensor panels, by eliminating the use of a substrate layer, or by substituting a substrate layer integrated in a display for a standalone substrate layer.